There are two general kinds of steels: carbon steel and alloy steel. Carbon steel contains only iron and carbon, while alloy steel contains some other alloys elements such as nickel（镍）, chromium（铬）, manganese（锰）, molybdenum（钼）, tungsten（钨）, vanadium（钒）, etc.
1.Carbon steels
1)Low carbon steel containing from 0.05 to 0.15 percent carbon; this steel is also known as machine steel.
2)Medium carbon steel containing from 0.15 to 0.60 percent carbon.
3)High carbon steel containing from 0.6 to 1.50 percent carbon; this steel is sometimes called “tool steel”（工具钢）
2.Alloy steels
1)Special alloy steels, such as nickel steel, chromium steel
2)High-speed steel also known as self-hardening steel
Carbon steels are the most common steels used in industry. The properties of these steels depend only on the percentage of carbon they contain. Low carbon steels are very soft and can be used for bolts and for machine parts that do not need strength.
Medium carbon steel is a better grade and stronger than low carbon steel. It is also more difficult to cut than low carbon steel.
High carbon steel may be hardened by heating it to a certain temperature and then quickly cooling in water. The more carbon the steel contains and the quicker the cooling is, the harder it becomes. Because of its high strength and hardness this grade steel may be used for tools and working parts of machines.
But for some special uses, for example, for gears（齿轮）, bearings（轴承）, springs（弹簧）, shafts（轴） and wire（金属丝）, carbon steels cannot be always used because they have no-properties needed for these parts.
Mechanical properties are the characteristics responses of a material to applied forces. The knowledge of mechanical properties of materials is very essential in order to construct a mechanically sound structure such as a bridge on the river. Mechanical properties can be determined by conducting experimental tests on the material specimen（样本）. Some important mechanical properties of materials are:
1)Strength(强度) (in tension(张力) ,compression(压r), shear（剪切力） and bending（弯曲力）)
2)Stiffness (刚性)
3)Ductility（延伸性）
4)Hardness (硬度）
5)Toughness (韧性)
1. Strength or Mechanical strength of a material may be defined as the ability of the material to sustain loads without undue（不适当） distorsion（扭转） or failure. Material should have adequate strength when subjected to tension, compression, shear, bending or torsion as per（按照）in the intended use.
For example the crankshaft of a automobile should have proper torsion strength.
2. Stiffness. Stiffness is the ability of a material or shape to resist elastic deflection. For identical shapes, the stiffness is proportional to modulus of elasticity. A material which deforms less under a given load is more stiff（硬） than the one which deforms（变形） more.
Vocabulary
1.carbon n. 碳、石墨
2.alloy n. 合金 v. 熔合t结）
3.alloying elements 合金元素
4.nickel n. 镍，Ni
[ ''krəumjəm ] n. 铬， Cr
6.manganese [.mæŋgə''ni:z, ] n. 锰
7.molybdenum [ mə''libdinəm ] n. 钼，Mo
8.construct vt. 构造（制、筑）
9.shear v.剪断 n. 剪切力
10.stiffness n. 刚性（度）
11.tungsten n. 钨
12.vanadium n. 钒
13.low carbon steel 低碳钢l
14.high carbon steel 高碳钢
15.high speed steel 高速钢
16.self-hardening steel 自硬钢；风钢
17.toughness n. 韧性、韧度
18.ductility n. 可延n型
19.sustain vt. 支撑、承受
20.undue a. 过渡的、过分的；不适当的
21.distortion n. 变形；扭曲
22.failure n. 失效（败、灵）
23.adequate a.足够的
24.deflection n. 偏转；挠曲
25.modulus n.模（数、量）
26.deform v. 使（变形）； 损坏
27.elasticity n. 弹力学；弹性变形
28.torsion n. 扭矩，转拒
